The invention pertains to automatic and semi-automatic hardness testing equipment and more particularly to testing instruments that measure hardness by subjecting the test parts to the impact of an indenter probe and then electrically sense the depth of indenter penetration.
In controlling the quality of production parts, there is often a need to measure, on a routine production basis, the hardness of materials, especially metals, that are incorporated into a manufactured article. For example, in the fabrication of aircraft parts made of various lightweight alloys of aluminum, the hardness of these alloys is tested to verify that the part is made of the proper mixture of alloyed components and has the desired heat treatment. More particularly, it has heretofore been discovered that a reliable correlation exists between the composition of an aluminum alloy having a given heat treatment (temper), and the combined hardness and the electrical conductivity of the alloy. Thus, by measuring both the hardness and the electrical conductivity of the production part or stock material, empirically generated tables can be referred to verify that the particular alloy has the proper mixture of alloyed metals, and the correct temper.
Known testing devices are available for individually measuring hardness and electrical conductivity pursuant to the above characterized quality control test. The present invention pertains to an improvement in a testing apparatus for measuring material hardness. Prior to the present invention, hardness has been measured by manual, automatic and semi-automatic testers. One popular tester of the manually actuated type provides an indenter pin slidably mounted in a handheld housing, and a displacement dial indicator also mounted on the housing and mechanically linked to the slidable indenter pin. The operator manually forces the tester, including the slidable indenter pin, against the face of the part to be tested, causing the indenter pin to penetrate the material. This type of tester is generally referred to as a single step indenter because the tested material is subjected to the impact of a single stroke of the indenter pin tip. The degree of penetration of the indenter pin is converted by the mechanical linkage to the indicator dial which measures the amplitude of the penetration, and hence provides an indication of the material hardness. In some versions, the penetration is measured by electromechanical means. In this kind of tester, the indenter pin has a specially contoured tip, which when used in a tester of this type, has proven to provide a usable degree of correlation between the dial indicated hardness and other more scientifically based, and more precise hardness scales such as those resulting from Rockwell, Brinell and scleroscope testing procedures and/or devices.
Although one of the other hardness testing procedures, such as Rockwell or Brinell would provide a more accurate and consistent measurement of hardness, the use of Rockwell, Brinell and other multiple step tests is undesirable for performing quality control testing of aluminum alloys on a production basis, where a large number of parts must be tested quickly and easily, by nonscientifically trained personnel.
For this purpose, manually actuated, one step indenter testers have been preferred for production testing. While faster, currently available one step indenter-type testers are deficient in terms of producing consistent and hence accurate hardness measurements. Moreover, when such a device is manually, as opposed to automatically actuated, operator fatigue has been found to enter into the accuracy of the testing, especially after an operator has tested a large number of parts and the manually dependent impact force on the indenter pin becomes erratic. The problem of achieving consistent results becomes even more acute, when the tester is employed to determine the hardness of materials that exhibit an impact rate dependent hardness. Some materials, such as aluminum and various aluminum alloys, show a marked change in hardness, as a function of the rate at which the indenter tip is driven toward and into the material. At an extremely high rate of impact, the creep of the material is minimal and hence the indenter tip does not penetrate the metal as much as when the impact rate is decreased to afford time for the material to yield as thetip penetrates into the test part.
Another shortcoming of available hardness testers, especially the one step, indenter impact-types, is the difficulty in maintaining an acceptable level of calibration of the tester over the long intervals of time that are typically associated with the testing of production parts. Frequent recalibration of the tester is required which involves keeping track of the number of test cycles that have occurred since the last calibration, and then at the end of that interval, stopping the testing procedure and recalibrating the device by an adjustment provided in the mechanical or electromechanical coupling between the indenter tip and the displacement indicator dial or readout. While frequent recalibration of hardness testers of this general type is probably unavoidable, the difficulty involved in readjusting these devices, not only slows the testing operation down, but in some cases requires that the recalibration be performed by different, more skilled personnel than those which are performing the routine production testing.
Accordingly it is an object of the invention to provide a one step, indenter impact-type hardness testing apparatus, suitable for use in the rapid testing of large numbers of production parts or materials, and is characterized by the capability of yielding consistent hardness measurements that are independent of variations in the indenter impact force usually associated with operator actuated indenter devices.
Still another object of the invention is to provide a hardness testing apparatus of the above characterized type capable of achieving consistent hardness measurements and which can be operated at a greater cyclic speed than is achievable by manually actuated hardness testers of this kind.
A further object of the invention is to provide a one step, indenter impact hardness tester which is automatically controlled so that after predetermined usage, a recalibration processor provides in a semi-automatic manner, for two different levels of recalibration, one of which is automatically implemented during succeeding test cycles to minimize the apparatus down time, and the other of which requires a shutdown of the apparatus for servicing.